1. Field of the Invention
The present invention relates to an apparatus and method for processing substrates. Specifically, the invention relates to a method for depositing a conformal layer of material on a substrate using physical vapor deposition process.
2. Background of the Related Art
Sub-quarter micron multi-level metallization represents one of the key technologies for the next generation of ultra large-scale integration (ULSI) for integrated circuits (IC). In the fabrication of semiconductor and other electronic devices, directionality of particles being deposited on a substrate is important to improve adequate in filling of electric features. As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to 0.25 xcexcm or less, whereas the thickness of the dielectric layer remains substantially constant. Thus, the aspect ratios for the features, i.e., the ratio of the depth to the minimum lateral dimension, increases, thereby pushing the aspect ratios of the contacts and vias to 5:1 and above. As the dimensions of the features decrease, it becomes even more important to get directionality in order to achieve conformal coverage of the feature sidewalls and bottoms.
Conventionally, physical vapor deposition (PVD) systems have been used to deposit materials in device features formed on a substrate. PVD systems are well known in the field of semiconductor processing for forming metal films. Generally, a power supply connected to a processing chamber creates an electrical potential between a target and a substrate support member within the chamber and generates a plasma of a processing gas in the region between the target and substrate support member. Ions from the plasma bombard the negatively biased target and sputter material from the target which then deposits onto a substrate positioned on the substrate support member. However, while such processes have achieved good results for lower aspect ratios, conformal coverage becomes difficult to achieve with increasing aspect ratios. In particular, it has been shown that coverage of the bottoms of the vias decreases with increasing aspect ratios.
One process capable of providing greater directionality to particles is ionized metal plasma-physical vapor deposition (IMP-PVD), also known as high density physical vapor deposition (HDP-PVD). Initially, a plasma is generated by introducing a gas, such as helium or argon, into the chamber and then biasing a target to produce an electric field in the chamber, thereby ionizing a portion of the gas. An energized coil positioned proximate the processing region of the chamber couples electromagnetic energy into the plasma to result in an inductively-coupled medium/high density plasma between the target and a susceptor on which a substrate is placed for processing. The ions and electrons in the plasma are accelerated toward the target by the bias applied to the target causing the sputtering of material from the target. Under the influence of the plasma, the sputtered metal flux is ionized. An electric field due to an applied or self-bias, develops in the boundary layer, or sheath, between the plasma and the substrate that accelerates the metal ions towards the substrate in a direction substantially parallel to the electric field and perpendicular to the substrate surface. The bias energy is preferably controlled by the application of power, such as RF, to the susceptor to attract the sputtered target ions in a highly directionalized manner to the surface of the substrate to fill the features formed on the substrate.
One of the problems with HDP-PVD processes is the inability to achieve conformal step coverage in the increasingly smaller device features. Conformal coverage of the bottoms and sidewalls of the features is needed to optimize subsequent processes such as electroplating. Electroplating requires conformal barrier and seed layers within the device features in order to ensure uniform filling of the feature. While conventional HDP-PVD achieves good bottom coverage due to the directionality of the ions provided by the bias on the substrate, the sidewall coverage can be less than conformal. This result is caused in part by the induced high directionality of ions towards the bottom of the features with little directionality toward the sidewalls.
The effects of a bias on film deposition on and into the features in/on a substrate can be described with reference to FIGS. 1-2 which illustrate the direction of metal ions 12 entering a via 16 formed on a substrate 10. FIG. 1 illustrates a DC magnetron PVD processing environment wherein no bias is supplied to the substrate 10 (the presence or absence of an applied bias being substantially irrelevant to traditional planar target DC sputtering). As a result, the directionality of the ions 12 is determined primarily by the ejection profile of material (usually atoms) from the target and by the inelastic collisions with other particles in the chamber, such as Ar ions which are provided in a plasma. The angular distribution 22 of the ions in FIG. 1 typically results in little deposition on the bottom 18 of the via 16. In addition to the angular distribution of the incoming ions 12, the feature dimensions also determine the resulting step coverage. Thus, where the feature opening is wider than the depth of the feature, deposition material can reach all surfaces of the feature for relatively uniform deposition. However, where the feature is narrow compared to the depth, the particles travelling substantially non-parallel to the feature depth deposit around the feature opening, resulting in less deposition at the bottom 18 of the via 16.
FIG. 2 illustrates the processing environment in a HDP-PVD process wherein the angular distribution of the ions 12 is influenced by the electrical field E due to interaction between the charged target material and the applied or self-bias at the surface of the substrate. The electric field E is oriented perpendicular to the substrate 10 and the positively charged ions 12 are influenced into a trajectory parallel to the electric field E toward the bottom 18 of the via 16. The angular distribution 23 of the ions 12 in FIG. 2 typically results in moderate to lower deposition on the sidewalls 20 and higher to moderate deposition on the bottom 18 than is possible without ionization of the sputtered material. As compared to the angular distribution 22 of FIG. 1, the distribution 23 exhibits a tighter distribution indicating more directionality parallel to the electric field E.
Therefore, there is a need to provide a technique for depositing a layer conformal over the surface of features, particularly sub-half micron and higher aspect ratio features.
The present invention generally provides an apparatus and method for depositing a conformal layer on device features in a plasma chamber by PVD. In one aspect of the invention, a chamber having a target, a power supply coupled to the target adapted to provide a signal having a desired waveform, a substrate support member, a power supply connected to the substrate support member, and a magnetic field generator is provided. The target comprises a material to be sputtered by a plasma formed adjacent to the target during processing. The signal supplied by the power supply coupled to the target preferably comprises a negative voltage portion and a zero-voltage portion. Preferably, the power supply connected to the substrate support member supplies a substantially constant negative bias to the substrate.
In another aspect of the invention, a plasma is supplied to a chamber to sputter a material from a target. A coil is energized proximate the chamber to enhance ionization of the sputtered material. During processing, a modulated signal is provided to the target. In one embodiment, the modulated signal is varied between a negative voltage portion during which the target material is sputtered onto a substrate and a zero-voltage portion during which the deposited material is re-sputtered from the substrate. A bias is provided to the substrate to influence the direction of ions in the chamber during processing.